Small particles entrained in a flowing stream of gas can be measured and counted by instruments which direct a beam of light through the stream of gas and detect the light scattered from individual particles entrained in the stream. The instruments provide a count of particles in each size range and thus measure the particle size distribution in the sample stream. Practical applications of these devices include use in measuring particle distribution in aerosols, monitoring of work place environments to ensure compliance with health and safety standards and the use in conjunction with clean room environments to insure that proper specifications are met in the manufacture of products which have rigid purity specifications.
Existing particle measuring instruments can accurately measure both the size and number of particles in a flowing stream under ideal conditions. However, when the concentration of particles entrained in a sample stream is great, the particle measuring instruments do not give accurate measurements of particle size distribution because individual particles cannot be discerned from the data generated by the photodetectors. The effective measurement limit of particle concentration for some commercially available particle measuring instruments is approximately to 1 million particles per cfm. For example, an instrument sold by the assignee of this invention and identified as the Hiac/Royco 5230 is estimated to be able to measure particles of various sizes up to concentrations of 1 million at 1 cfm.
In order to measure gas samples that have concentrations near and over 1 million particles per cfm and avoid the problems associated with this condition, the sample can be diluted with a known volume of gas or air having no particles. The actual concentration and composition of the unknown sample can then be calculated by applying ratio of dilution to the measurement of the diluted sample.
When diluting a sample it is important to ensure that the flow dynamics do not affect the particle size distribution in the diluted sample. In addition, the flow should be maintained in a turbulent state to keep the particles suspended in the gas. With improper flow dynamics, the ratio of large particles measured to small particles measured can become skewed and application of the ratio of dilution to the data will not accurately reflect the size concentrations of particles present in the undiluted stream. If the turbulence is not maintained in the system, larger particles entrained in the stream will settle out and the accuracy of the determination of the concentration of the larger particles will be reduced.
Attempts to dilute an air sample by using a collection device in the form of a small entrance port having a small opening, such as a needle valve and mixing the collected sample with filtered air has been unsatisfactory. The flow dynamics of the air entering a narrow opening affect the particle distribution because larger particles are excluded from or drop out of the sample stream passing through the restricted opening. Accordingly, the resultant data generated does not accurately reflect the particle size distribution suspended in the original gas sample.